In order to increase the throughput of printers producing multilayer images, single-pass, multiple exposure systems have been developed. A variety of methods are known for creating multiple Raster Output Scanner (ROS) single-pass color printer architectures. A desirable configuration, because of the high throughput achievable, is the tandem system where multiple exposures are simultaneously produced on separate photoreceptors. For example, U.S. Pat. No. 4,370,047 to Damouth et al. discloses, in FIG. 4, a tandem architecture utilizing four scanning systems with a single polygon scanner.
It is also common in single-pass ROS systems to utilize a single photoreceptor whereby a plurality of primary color images may be developed sequentially or on top of one another, to achieve a color image which is transferable to an output medium or copy sheet in a conventional manner. For example, U.S. Pat. No. 4,791,452 to Kasai et al. illustrates such a printing system.
Common to all of these systems is the need to accurately monitor and control the exposure source, for example, a radiant energy beam or laser, which is used to selectively expose regions of the photoreceptor, with respect to the photoreceptor motion. For example, U.S. Pat. No. 4,349,847 to Traino and "Printer Motion Compensation" by Hull et al., published in the Xerox Disclosure Journal, Vol. 14, No. 2, p. 85, illustrate methods by which the exposure of an image raster is controlled with respect to the photoreceptor. Furthermore, the registration of sequential color images, in both the fast-scan direction (direction of the traversing laser beam) and the slow-scan direction (process direction of the photoreceptor), relies on the control of the ROS motion with increasing accuracy as color rendition requirements become more stringent for color printers.
Heretofore, various techniques have been employed to monitor or adjust the location of laser beams, including those used in raster input scanners (RISs) and ROSs. Some of the approaches used are illustrated in the following disclosures which may be relevant:
"Lateral-Effect Photodiodes" by Kelly, published in Laser Focus, March 1976, discloses the use of lateral-effect photodiodes for determination of the displacement of a light spot which impinges on the active area of the photodiode. As described in the technical disclosure, a typical circuit utilizes a differential amplifier and a summing amplifier to determine the light spot position. Also noted are the major disadvantages of the lateral-effect detector, that of slow speed and high noise.
U.S. Pat. No. 4,518,855 Patentee: Malak Issued: May 21, 1985 PA1 U.S. Pat. No. 4,827,120 Patentee: Stauffer Issued: May 2, 1989 PA1 U.S. Pat. No. 4,926,050 Patentee: Shemwell Issued: May 15, 1990 PA1 U.S. Pat. No. 4,981,354 Patentee: DeHainaut et al. Issued: Jan. 1, 1991 PA1 U.S. Pat. No. 4,987,293 Patentee: Baciak Issued: Jan 22, 1991 PA1 U.S. Pat. No. 5,043,744 Patentee: Fantuzzo et al. Issued: Aug. 27, 1991
The relevant portions of the foregoing patents may be briefly summarized as follows;
U.S. Pat. No. 4,518,855 by Malak discloses a method and apparatus for checking and monitoring the linear alignment of two shafts where one shaft is the reference shaft and the other is aligned thereto. Generally, a pair of dual-axis (x-y) detectors and associated radiation beams are mounted with respect to the shafts to be aligned. Signals produced by the detectors are used by a readout means to indicate when the defined alignment condition has been achieved.
U.S. Pat. No. 4,827,120 by Stauffer discloses an elongated, cylindrical shaped element having an exposure window along the length of the cylinder. Light entering the cylinder, via the exposure window, is diffused towards the ends thereof where photodiodes are used to produce electrical signals in response to the light. The light impinging on the photodiodes, and therefore the output signals of the photodiodes, is proportional to the location at which the light enters the cylinder along the length of the exposure window.
U.S. Pat. No. 4,926,050 by Shemwell discloses a distance measuring system having a receiver which employs a pair of spaced apart, lateral effect cells which produce signals in response to a laser beam traversing the surface of the cells. The transmitter repeatedly swings a well defined beam across an arc which covers the location of the receiver, whereby the angular velocity of the beam is a constant. Hence, measurement of the amount of time required for the beam to traverse the distance between the cells allows the determination of the distance between the transmitter and the receiver.
U.S. Pat. No. 4,981,354 by DeHainaut et al. discloses a differential tilt sensor intended for use with a phased array telescope. A two-dimensional lateral effect cell is used to measure the centroid locations of a pair of chopped beams, the output of the cell being subsequently differentiated to produce both x and y analog tilt output signals.
U.S. Pat. No. 4,987,293 by Baciak discloses a digital position monitor which is used to output the x and y position of a laser beam impinging upon a lateral effect detector, the x and y positions being used to control a phased array telescope. The two-channel system (x, y) comprise an analog-to-digital converter, a microprocessor, and a digital-to-analog converter connected in series to provide the x and y outputs. The position of the laser beam output by the laser telescope is constantly monitored to enable the control thereof, and to effectuate the steering of a synthetic array of multiple telescopes.
U.S. Pat. No. 5,043,744 by Fantuzzo et al. discloses a raster output scanner (ROS) in which a movable scanning element scans a high intensity beam across a movable imaging element to record images thereon. The movement of the imaging member and the tilt of the high intensity beam are controlled with respect to one another in order to produce a multi-color electrophotographic image which is transferred to a copy sheet. Timing marks, located in the marginal region of the photoreceptor, are used to provide the basic synchronization between the high intensity ROS beam and the movement of the photoreceptor, based upon pulses produced in response to the reflection, or non-reflection of the high intensity beam.
In accordance with the present invention, an apparatus is provided for sensing the position of a radiant energy beam in a first direction, the apparatus includes a lateral-effect photodiode having at least two electrodes spaced in opposition to one another so as to produce electrical signals in response to the radiant energy beam impinging upon an active region of the photodiode. In addition, the electrodes each have integrating means to integrate the electrical signal produced during a portion of the period in which the radiant energy beam impinges upon the photodiode, the output of the integrating means being subsequently passed to differentiating means. The differentiating means determines the position of the radiant energy beam as a function of the difference between the integrated electrical signals.
In accordance with an additional aspect of the present invention, there is provided an apparatus suitable for sensing the position of a radiant energy beam traversing a surface of the apparatus in a first direction, and for indicating the time at which the beam traverses a locus defined therein. The apparatus includes a lateral-effect photodiode having a first pair of electrodes, spaced apart from one another, for producing electrical currents in response to carriers of a first polarity produced by the radiant energy beam impinging upon an active photodiode region lying between the first pair of electrodes. The lateral-effect photodiode also includes a second pair of electrodes, spaced apart from the first electrode pair, which are adjacent and parallel to one another, extending the length of the active photodiode region. The second pair of electrodes also output electrical signals in response to carriers of a second polarity produced by the radiant energy beam impinging upon the active photodiode region which at least partially covers the second electrode pair.
In accordance with another aspect of the present invention, an imaging apparatus is provided for forming, during a single pass, multiple image exposure frames on a photoconductive member moving in a process direction. Included is a Raster Output Scanner (ROS) unit for generating image-modulated scan lines, or rasters, which are transmitted along an optical path to an associated image exposure frame. The ROS unit includes a laser, means for modulating the intensity of the laser beam in response to input video signals, and optics for directing the laser beam onto the associated image exposure frame of the photoconductive member to provide an optically focused scan line at the photoreceptor surface. Also included in the imaging system is a sensing element for sensing the position of the laser scan line with respect to the process direction and for producing a signal as a function of the scan line position. In addition, a beam steering mechanism, responsive to the signal produced by the sensing element, is provided for altering the position of the scan line to enable steering of the scan line in the process direction.
In accordance with yet another aspect of the present invention, there is provided a method for forming multiple image exposure frames on a photoconductive member moving in a process direction during a single pass. The method begins with the step of generating an image-modulated scan line with the ROS beam. Next, the position of the beam is monitored by a sensing circuit which employs a lateral-effect photodiode having at least two opposing electrodes. The current generated in the photodiode electrodes is integrated during a period in which the beam impinges upon the active region of the photodiode. Subsequently, the position of the scan line is determined as a function of the ratio between a sum and a difference signal determined from the integrated signals. The ratio is then used by a steering mechanism to adjust the transit path of the scan line at the photoreceptor in response to the ratio.